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Abstract:

According to one embodiment, a method for cleaning a semiconductor
substrate comprises supplying water vapor to a surface of a semiconductor
substrate on which a concave-convex pattern is formed while heating the
semiconductor substrate at a predetermined temperature, cooling the
semiconductor substrate after stopping the heating and the supply of the
water vapor and freezing water on the semiconductor substrate, after
freezing the water, supplying pure water onto the semiconductor substrate
and melting a frozen film, and after melting the frozen film, drying the
semiconductor substrate.

Claims:

1. A method for cleaning a semiconductor substrate, comprising: supplying
water vapor to a surface of a semiconductor substrate on which a
concave-convex pattern is formed while heating the semiconductor
substrate at a predetermined temperature; cooling the semiconductor
substrate after stopping the heating and the supply of the water vapor
and freezing water on the semiconductor substrate; after freezing the
water, supplying pure water onto the semiconductor substrate and melting
a frozen film; and after melting the frozen film, drying the
semiconductor substrate.

2. The method for cleaning a semiconductor substrate according to claim
1, wherein the water vapor includes isopropyl alcohol.

3. The method for cleaning a semiconductor substrate according to claim
2, wherein concentration of the isopropyl alcohol is 5 to 30 wt %.

4. The method for cleaning a semiconductor substrate according to claim
1, wherein the semiconductor substrate is cooled to -40.degree. C. or
lower to freeze water on the semiconductor substrate.

5. The method for cleaning a semiconductor substrate according to claim
1, wherein an adsorbed water layer is formed on a surface of a particle
on a convex pattern by supplying the water vapor.

6. The method for cleaning a semiconductor substrate according to claim
1, wherein the water vapor is supplied while the semiconductor substrate
is being heated so that a surface temperature of the semiconductor
substrate is 40.degree. C. to 80.degree. C.

7. The method for cleaning a semiconductor substrate according to claim
1, wherein a convex portion of the concave-convex pattern is formed
between a wide concave portion and a narrow concave portion.

8. A device for cleaning a semiconductor substrate, comprising: a
substrate holding unit holding a semiconductor substrate on which a
concave-convex pattern is formed and rotates the semiconductor substrate;
a first supply unit supplying water vapor to the semiconductor substrate
held by the substrate holding unit; a heating unit heating the
semiconductor substrate to a predetermined temperature while the first
supply unit is supplying water vapor to the semiconductor substrate; a
cooling unit freezing water attached to a surface of the semiconductor
substrate held by the substrate holding unit; and a second supply unit
supplying pure water to the surface of the semiconductor substrate held
by the substrate holding unit and melting a frozen film on the
semiconductor substrate.

9. The device for cleaning a semiconductor substrate according to claim
8, wherein the water vapor includes isopropyl alcohol.

10. The device for cleaning a semiconductor substrate according to claim
9, wherein concentration of the isopropyl alcohol is 5 to 30 wt %.

11. The device for cleaning a semiconductor substrate according to claim
8, wherein the cooling unit cools the semiconductor substrate to
-40.degree. C. or lower to freeze water.

12. The device for cleaning a semiconductor substrate according to claim
8, wherein the heating unit heats the semiconductor substrate so that a
surface temperature of the semiconductor substrate is 40.degree. C. to
80.degree. C.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims benefit of priority from
the Japanese Patent Application No. 2011-206009, filed on Sep. 21, 2011,
the entire contents of which are incorporated herein by reference.

FIELD

[0002] Embodiments described herein relate generally to a method and a
device for cleaning a semiconductor substrate.

BACKGROUND

[0003] With increasing miniaturization of device pattern due to
advancements in semiconductor manufacturing technologies, a cleaning
technique for removing minute contaminant particles from semiconductor
substrate is desired.

[0004] A conventional two-fluid jet cleaning device used as a single wafer
cleaning device removes particles by rotating a wet semiconductor
substrate and spraying mist-like droplets (droplet mist) generated by
mixing gas such as dry air or nitrogen and liquid such as pure water to
the surface of the semiconductor substrate.

[0005] A freeze cleaning process is known as a method for removing
particles on a semiconductor substrate. In a conventional freeze cleaning
process, first, pure water is supplied on a semiconductor substrate and a
part of the pure water is splashed out of the substrate by rotating the
semiconductor substrate, so that a liquid film (water film) is formed on
the semiconductor substrate. Then, coolant gas is discharged to the
semiconductor substrate to freeze the liquid film and particles are
captured by a frozen film (ice film) by using volume expansion force
generated when the phase of the liquid film changes from liquid to solid.
Thereafter, pure water is supplied to the semiconductor substrate to melt
the frozen film and the particles are discharged from the semiconductor
substrate along with the pure water.

[0006] However, in such a conventional freeze cleaning process, there is a
problem that fine patterns on the semiconductor substrate are damaged by
the volume expansion force of the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a flowchart for explaining a freeze cleaning process by a
comparative example;

[0008] FIG. 2 is a graph showing a relationship among a particle removal
rate, the number of pattern damages, and cooling temperature in the
comparative example;

[0009] FIG. 3 is a diagram showing portions of pattern damage in the
comparative example;

[0010] FIG. 4 is a diagram for explaining forces applied to a pattern in
the comparative example;

[0011] FIG. 5 is a diagram showing removal tendencies of particles in the
comparative example;

[0012] FIG. 6A is a vertical cross-sectional view showing a particle
attached to a planar portion in the comparative example;

[0013] FIG. 6B is a top view showing a particle attached to a planar
portion in the comparative example;

[0014] FIG. 6C is a vertical cross-sectional view showing a particle
attached to convex portions in the comparative example;

[0015] FIG. 6D is a top view showing a particle attached to convex
portions in the comparative example;

[0016] FIG. 7 is a schematic configuration diagram of a cleaning device
for semiconductor substrate according to an embodiment of the present
invention;

[0017] FIG. 8 is a schematic configuration diagram of a holding heating
unit according to a modified example;

[0018] FIG. 9 is a flowchart for explaining a method for cleaning a
semiconductor substrate according the embodiment;

[0019] FIG. 10A is a diagram showing a liquid film formed on a substrate
by the freeze cleaning process of the comparative example;

[0020] FIG. 10B is a diagram showing an adsorbed water layer formed on the
substrate by the cleaning method of the embodiment; and

[0021] FIG. 11 is a diagram showing an adsorbed water layer formed on a
surface of a particle on a convex pattern.

DETAILED DESCRIPTION

[0022] According to one embodiment, a method for cleaning a semiconductor
substrate comprises supplying water vapor to a surface of a semiconductor
substrate on which a concave-convex pattern is formed while heating the
semiconductor substrate at a predetermined temperature, cooling the
semiconductor substrate after stopping the heating and the supply of the
water vapor and freezing water on the semiconductor substrate, after
freezing the water, supplying pure water onto the semiconductor substrate
and melting a frozen film, and after melting the frozen film, drying the
semiconductor substrate.

[0023] Prior to the description of an embodiment of the present invention,
the circumstances how the inventors came up with the invention will be
described. FIG. 1 is a flowchart for explaining a freeze cleaning process
by a comparative example. In the freeze cleaning process by the
comparative example, first, pure water is supplied on a semiconductor
substrate (step S1) and a part of the pure water is splashed out of the
substrate by rotating the semiconductor substrate, so that a thin liquid
film (water film) is formed on a surface of the semiconductor substrate
(step S2). Next, coolant gas is discharged to the semiconductor substrate
to freeze the liquid film and a particle is captured by a frozen film
(ice film) by using volume expansion force generated when the phase of
the liquid film changes from liquid to solid (step S3). Then, pure water
is supplied to the semiconductor substrate to melt the frozen film (step
S4) and the particle is discharged from the semiconductor substrate along
with the pure water by spin-drying (step S5).

[0024] FIG. 2 is a graph showing a relationship among a particle removal
rate, the number of pattern damages in a device pattern having a fine
concave-convex structure, and cooling temperature in step S3 when the
freeze cleaning process described above is performed. As shown in FIG. 2,
the lower the freezing temperature, the higher the particle removal rate.
This is because when the freezing temperature is lowered, ice crystals
grow and the volume expansion rate increases. However, as shown in FIG.
2, as the freezing temperature is lowered, the number of pattern damages
increases. Therefore, in the freeze cleaning process as shown in FIG. 1,
it is found that the particle removal rate cannot be improved while the
fine concave-convex pattern is prevented from being damaged.

[0025] The inventors closely investigated generation status of the pattern
damages to solve the problem of the pattern damages described above. FIG.
3 shows a map indicating defective portions in which pattern collapse
occurs in a 300 mm wafer and SEM (scanning-type electron microscope)
photographs of the collapsed and damaged patterns. As seen from the SEM
images, the portions in which pattern damage (error) is shown are almost
convex portions at an end of a dense pattern and there is a wide concave
portion next to the convex portion.

[0026] As shown in FIG. 4, a convex portion 601 at an end of a dense
pattern is located between a wide concave portion 602 and a narrow
concave portion 603 in the dense pattern. The concave portion 602 and the
concave portion 603 have different amounts of water to be frozen, so that
it can be said that they have different volume expansion amounts of ice
when the water is frozen. Therefore, the volume expansion amount of ice
formed in the concave portion 602 increases, so that it can be said that
a strong force is applied to the convex portion 601 to collapse the
convex portion 601 and damage occurs. To prevent such pattern damage from
occurring, it is required to reduce the amount of water frozen in the
concave portion.

[0027] Further the inventors systematically investigated particles that
were removed and particles that were not removed when the freeze cleaning
process described above was performed. As shown in FIG. 5, the particles
that were removed did not tend to be attached to a dense pattern portion,
but tended to be attached to a planar portion. On the other hand, the
particles that were not removed tended to be attached to convex portions
In which fine patterns were densely located. There are characteristics
that the removal tendency of particle is different depending on a portion
where the particle is attached. It is considered that this is caused by
the difference of the volume expansion force of the water when the water
is frozen, which is applied to the particle. As shown in FIG. 6B, a
particle 610 attached to a planar portion 611 as shown in FIG. 6A
receives a volume expansion force of the water when the water is frozen
at a circle portion 613 around a contact point 612 that is in contact
with the planar portion 611. On the other hand, as shown in FIG. 6D, a
particle 620 attached to convex portions 621 as shown in FIG. 6C receives
a volume expansion force of the water when the water is frozen at very
small points 623 near contact points 622 between the particle 620 and the
convex portions 621. It is considered that the particle 620 is difficult
to be removed because the volume expansion force which the particle 620
on the convex portions 621 receives is small. FIGS. 6A and 6C are
vertical cross-sectional views. FIGS. 6B and 6D are top views.

[0028] The embodiment described below can solve the above problem.
Hereinafter, the embodiment of the present invention will be described
with reference to the drawings.

[0029] FIG. 7 shows a schematic configuration of a cleaning device for
semiconductor substrate according to the embodiment of the present
invention. The cleaning device includes a holding heating unit 100, a
vapor supply unit 200, a cooling unit 300, and a hot water supply unit
400.

[0030] The holding heating unit 100 includes a spin cup 101, a rotation
shaft 102, a spin base 103, chuck pins 104, and a heater 110. The
rotation shaft 102 extends in a substantially vertical direction. The
spin base 103 having a disk shape is attached to the upper end of the
rotation shaft 102. The rotation shaft 102 and the spin base 103 can be
rotated by a motor not shown in FIG. 7.

[0031] The chuck pins 104 are provided on a circumferential portion of the
spin base 103. The chuck pins 104 sandwiches a substrate (wafer) W, so
that the holding heating unit 100 can hold the substrate W substantially
horizontally and rotate the substrate W.

[0032] When hot pure water is supplied from the hot water supply unit 400
to an area near the rotation center of the surface of the substrate W,
the liquid spreads in radial directions of the substrate W. The holding
heating unit 100 can spin-dry the substrate W. Useless liquid scattering
in the radial directions of the substrate W is captured by the spin cup
101 and discharged through a waste liquid pipe 105.

[0033] The heater 110 can heat the substrate W to a desired temperature.
As the heater 110, for example, a heating stage of resistance heating
type, which is integrated into the spin base 103, can be used. The heater
110 only has to maintain the substrate W at a constant temperature. The
heater 110 has not necessarily to be integrated into the spin base 103.
The heater 110 may be a heating mechanism that supplies hot water or the
like.

[0034] The vapor supply unit 200 supplies vapor to the substrate W held by
the holding heating unit 100. The vapor supply unit 200 has pipes 201 to
203 and a vaporizer 204. The vaporizer 204 vaporizes pure water supplied
through the pipe 201 by using carrier gas supplied through the pipe 202
and generates water vapor. The carrier gas is, for example, nitrogen. The
water vapor generated by the vaporizer 204 is supplied to the substrate W
from a nozzle N through the pipe 203.

[0035] The cooling unit 300 has a pipe 301 for supplying a cooling solvent
such as liquid nitrogen. The cooling solvent is supplied from the nozzle
N to the substrate W. The cooling unit 300 may use cooling gas such as
liquid nitrogen or may perform indirect cooling using a liquid
refrigerant.

[0036] The hot water supply unit 400 has a pipe 401 for supplying hot pure
water. The hot pure water is supplied from the nozzle N to the substrate
W. The hot pure water is, for example, pure water of about 50° C.
The hot water supply unit 400 may supply not only the hot pure water, but
also pure water of room temperature.

[0037] The holding heating unit 100 and the nozzle N are provided in a
process chamber not shown in FIG. 7.

[0038] Although the cleaning device shown in FIG. 7 has a configuration in
which the water vapor, the cooling solvent, and the hot pure water are
supplied from the same nozzle N, the cleaning device may have three
nozzles so that the water vapor, the cooling solvent, and the hot pure
water are supplied from the three nozzles respectively, or may have two
nozzles so that the water vapor and the hot pure water are supplied from
one nozzle and the cooling solvent is supplied from the other nozzle.

[0039] As shown in FIG. 8, the holding heating unit 100 may be provided
with a function for lifting the substrate W, and when supplying the
cooling solvent to the substrate W, the substrate W may be lifted and
located near the nozzle N.

[0040] A method for cleaning a semiconductor substrate by using such a
cleaning device will be described using a flowchart shown in FIG. 9.
Operations of the holding heating unit 100, the vapor supply unit 200,
the cooling unit 300, and the hot water supply unit 400 can be controlled
by a control unit not shown in the drawings.

[0041] (Step S101) The semiconductor substrate W on which a concave-convex
pattern is formed is carried in by a conveying unit (not shown in the
drawings) and held by the holding heating unit 100. The concave-convex
pattern is formed by, for example, an RIE (Reactive Ion Etching) method
and there are particles such as process residues on the semiconductor
substrate W.

[0042] (Step S102) While the semiconductor substrate W is heated using the
heater 110, water vapor is supplied from the vapor supply unit 200 to the
surface of the semiconductor substrate W. The process chamber is filled
with a humidified atmosphere and water is adsorbed onto the surface of
the semiconductor substrate W. The semiconductor substrate W is
maintained at a predetermined temperature by the heater 110. Under such
an environment, water vapor is in an equilibrium state between adsorption
and desorption on the semiconductor substrate W, so that the water vapor
cannot be adsorbed onto the semiconductor substrate W exceeding the
saturated water vapor concentration. At room temperature, the water vapor
supplied to the semiconductor substrate W is condensed in the
concave-convex pattern on the semiconductor substrate W and a liquid film
is formed, so that the heater 110 is controlled to maintain the surface
temperature of the semiconductor substrate W between 40° C. and
80° C. and the amount of adsorbed water is adjusted. Thereby, an
adsorbed water layer suited to the freeze cleaning process is formed on
the surface of the semiconductor substrate W.

[0043] In the concave-convex pattern on the semiconductor substrate W,
various types of films, such as a CVD-SiO2 film, a coated SiO2
film, a CVD-SIN film, a polysilicon film, a boron-doped polysilicon film,
a tungsten film, are mixed, so that the amount of adsorbed water varies
depending on presence or absence of reaction (for example, hydrogen
bonding reaction) with the water adsorbed onto the surface. Therefore,
the surface temperature of the semiconductor substrate W needs to be
adjusted according to the state of the concave-convex pattern and the
state of films exposed on the surface.

[0044] (Step S103) The supply of water vapor from the vapor supply unit
200 and the heating by the heater 110 are stopped. Immediately after the
heating is stopped, a cooling solvent is supplied to the surface of the
semiconductor substrate W from the cooling unit 300 and the water
adsorbed onto the surface of the semiconductor substrate W is frozen. It
is possible to separate a particle from the semiconductor substrate W at
a portion where the particle is in contact with the semiconductor
substrate W by using a volume expansion force generated when the phase of
the water on the semiconductor substrate W changes from liquid to solid.
In this step, only the water adsorbed onto the surface of the
semiconductor substrate W and the water adsorbed onto the surface of the
particle are frozen, so that the water that covers the entire concave
pattern is not frozen. The surface of the semiconductor substrate W is
cooled to, for example, -40° C. or lower.

[0045] (Step S104) The hot water supply unit 400 supplies hot pure water
to an area near the rotation center of the surface of the semiconductor
substrate W and the holding heating unit 100 rotates the semiconductor
substrate W. The hot pure water receives centrifugal force generated by
the rotation of the semiconductor substrate W and spreads over the entire
surface of the semiconductor substrate W. Thereby, the frozen film melts.
After supplying the hot pure water, pure water at room temperature may be
supplied onto the semiconductor substrate W.

[0046] (Step S105) A drying process of the semiconductor substrate W is
performed. For example, the rotation speed of the semiconductor substrate
W is raised to a predetermined spin-dry rotation speed to perform a
spin-dry process in which the pure water remaining on the surface of the
semiconductor substrate W is thrown out and the semiconductor substrate W
is dried. The particles on the semiconductor substrate W are removed from
the semiconductor substrate W along with the pure water and the frozen
film melted in step S104.

[0047] In the freeze cleaning process by the comparative example shown in
FIG. 1, liquid water is supplied onto the semiconductor substrate, the
amount of water on the semiconductor substrate is gradually reduced by
rotating the semiconductor substrate, and a liquid film is formed. In
this method, when reducing the amount of water on the semiconductor
substrate, water in concave portions in the concave-convex pattern cannot
be removed, so that, as shown in FIG. 10A, a liquid film 701, in which
the concave-convex pattern is filled with water, is formed.

[0048] On the other hand, in the present embodiment, in step S102, instead
of supplying the liquid water, under a humidified atmosphere, an adsorbed
water layer is formed on the surface of the semiconductor substrate W by
gradually increasing an amount of adsorbed water to an equilibrium state
between adsorption and desorption of the water based on the temperature
of the surface of the semiconductor substrate W and the material of the
concave-convex pattern. Therefore, as shown in FIG. 10B, it is possible
to form an adsorbed water layer 702 in which the water is adsorbed only
onto the surface of the concave-convex pattern, so that it is possible to
reduce the amount of water frozen in the concave portion in the freezing
process in step S103.

[0049] In this way, a state is formed in which the concave-convex pattern
is not filled with water, so that, even if the freezing temperature is
lowered, it is possible to extremely reduce a force which is generated by
a difference between the volume expansion amounts of ices frozen in the
concave portion 704 and the concave portion 705 and which is applied to a
convex portion 703 located at an end of a dense pattern and between a
wide concave portion 704 and a narrow concave portion 705 of the dense
pattern. Therefore, it is possible to prevent damage from occurring.

[0050] Further, according to the cleaning method of the present
embodiment, as shown in FIG. 11, it is also possible to form an adsorbed
water layer 802 on the surface of a particle 801 on a convex pattern 803.
Therefore, when the water is frozen, a sufficient volume expansion force
can be obtained, so that it is possible to easily discharge the particle
801 along with the melted pure water from the semiconductor substrate by
melting and spin-drying the frozen film. Therefore, the particle removal
rate can be improved.

[0051] As described above, according to the present embodiment, it is
possible to effectively remove the particles on the semiconductor
substrate by the freeze cleaning while preventing damage from being
applied to the fine patterns.

[0052] In the above embodiment, IPA (isopropyl alcohol) may be added to
the pure water supplied to the vaporizer 204 through the pipe 201 so that
the concentration is about 5 to 30 wt %. Thereby, the surface tension is
reduced and the adsorbed water infiltrates immediately below the
particles, so that it is possible to further improve the particle removal
rate. Note that when the concentration of the IPA is raised higher than
the above-mentioned concentration, the surface tension is further
reduced, and there is a risk that the amount of frozen water decreases
and the particle removal rate degrades.

[0053] While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to limit
the scope of the inventions. Indeed, the novel methods and systems
described herein may be embodied in a variety of other forms;
furthermore, various omissions, substitutions and changes in the form of
the methods and systems described herein may be made without departing
from the spirit of the inventions. The accompanying claims and their
equivalents are intended to cover such forms or modifications as would
fall within the scope and spirit of the inventions.

Patent applications by Hiroshi Tomita, Yokohama-Shi JP

Patent applications by Minako Inukai, Yokohama-Shi JP

Patent applications in class Including work heating or contact with combustion products

Patent applications in all subclasses Including work heating or contact with combustion products